The use of inorganic whiskers and fibers to reinforce glasses, glass-ceramics, sintered ceramics, plastics, and metals has long been practiced. While the distinctions between fibers and whiskers are not always clearly delineated in the literature, whiskers have been characterized as relatively short, single-crystal fibers of small (&lt;100 microns) diameter, normally used as a dispersed phase in a selected matrix. In contrast, fibers are deemed to be multicrystalline or amorphous, and are frequently used in elongated, woven or otherwise interlocking, bundles, tows, or cloth as a reinforcement for a selected matrix.
Extensive study to understand the basic means underlying the strengthening improvement to composite bodies imparted by fibers has indicated the mechanism to be that of load transfer by the matrix to the fibers through shear. This load transfer shifts stress to the relatively long, high modulus fibers, and the fibers may additionally act to impede crack propagation in the matrix.
The basic strengthening mechanism is believed to be the same in whisker-containing composites, but the amount of load transferred by the matrix to the whiskers is dependent upon the length and aspect ratio of the whisker. Hence, in theory, shorter whiskers would not be loaded to the breaking stress and, consequently, full advantage could not be taken of their reinforcing capabilities.
Among the fibers and whiskers which have been demonstrated as reinforcing agents in numerous metal and non-metal matrices are those of silicon carbide (SiC). For example, U.S. Pat. No. 4,324,843 records the formation of SiC fiber reinforced glass-ceramic composite bodies wherein the glass-ceramic matrix is selected from the composition systems of aluminosilicate, lithium aluminosilicate, magnesium aluminosilicate, and combinations thereof. U.S. Pat. No. 4,464,475 discloses the production of SiC fiber reinforced glass-ceramic composite bodies wherein barium osumilite constitutes the predominant crystal phase. U.S. Pat. No. 4,464,192 describes the preparation of SiC whisker reinforced glass and glass-ceramic composite bodies wherein the glass-ceramic matrix is selected from the group of lithium aluminosilicate, magnesium aluminosilicate, aluminosilicate, and combinations thereof.
The above matrices are asserted to be suitable for use temperatures up to about 1300.degree. C. Above that temperature range those compositions are not refractory enough to provide a viscosity sufficiently high to transfer load to reinforcing fibers and whiskers. Consequently, the matrix deforms excessively and the composite suffers loss of load-bearing ability.
For applications requiring strength and stability at very high temperatures, wholly ceramic matrix materials, i.e., crystalline materials free of residual glassy phases, will be needed. These materials are more difficult to reinforce with fibers or whiskers, and the matrix-whisker interactions which govern the properties of the composites are not well understood. Improved dimensional stability has been postulated on the theory that, when dispersed in a crystalline matrix, whiskers will occupy sites along the grain boundaries of the crystals, and could significantly improve the creep resistance of the material. This would be due, for example, to an increase in the length of shear required and/or the added complexity of shear required to yield apparent creep.
It has also been postulated that the high elastic modulus and tensile strength of selected whiskers might enable them to produce composite products demonstrating superior strength-to-weight and stiffness-to-weight properties. For example, whiskers prepared from very stiff, low density covalent compounds such as carbides, nitrides, and oxides can exhibit elastic moduli higher than most metals, and are often many times stronger than steel when considered in proportion to their weight. Of course, the importance of these factors diminishes as matrix ceramics of inherently high elastic modulus are selected for strengthening.
An essential requirement of new ceramic materials for high temperature applications will be toughness, i.e., improved resistance of the material to cracking failure from flaws sustained in use. A material which can exhibit improved resistance to crack growth will provide increased fatigue lifetime and, desirably, a non-catastrophic failure mode which can be easily detected by routine inspection.
The mechanisms of toughening in wholly ceramic matrices have been reviewed by R. W. Rice in "Mechanisms of Toughening in Ceramic Composites", Ceram. Eng. Sci. Proc., 2(7-8) 661-701 (1981). Major strengthening mechanisms for fibers in these ceramics include load transfer, prestressing, crack impediment, crack deflection, and fiber pullout. Also noted, however, is the fact that second phases incorporated in composites for purposes of reinforcement provide many potential sources and preferred paths for localized stresses and crack growth. Thus some composites may have significantly lower compressive strengths than the pure ceramic matrix itself, or may suffer damage under compressive loading which leads to reductions in tensile strength.
Some of these strengthening mechanisms have been the subject of further theoretical analysis. In the case of crack deflection, for example, a theoretical paper by K. T. Faber et al., "Crack Deflection Processes--I. Theory", Acta Metall., 31 (4) 565-576 (1983) suggests that toughening by a factor of four ought to be attainable in crystalline ceramic composites containing small, rod-shaped particles by crack deflection alone, provided sufficient volume fractions of high aspect ratio particles are present. This is of course well beyond the net toughening effect yet observed in any wholly crystalline, whisker-containing ceramic composites produced up to the present time.
ZrO.sub.2 -based ceramics have been extensively utilized in high temperature applications in the glass and steel processing industries because of the very high refractoriness of this material. However, for the fabrication of structural components in high temperature environments, substantial increases in the toughness of such ceramics i.e., in their resistance to crack propagation under stress, would be required.
The need for tougher ZrO.sub.2 and other ceramics for high temperature engineering applications of this nature is discussed in the literature. In "Strengthening Strategies for ZrO.sub.2 -Toughened Ceramics at High Temperatures", N. Claussen, Materials Science and Engineering 71 (1985) 23-38, the author reviews a number of different ceramic systems, and proposes several possible strategies by which high temperature mechanical properties might be improved in these systems. Included among the proposed studies is that of adding SiC and/or Al.sub.2 O.sub.3 fibers or whiskers to reinforce matrix materials such as mullite, cordierite and tetragonal zirconia bodies.
Improved toughness has been attained in ZrO.sub.2 -containing composites formed by impregnating SiC fiber roving with partially stabilized zirconia slurries, as reported by B. A. Bender et al in Ceramic Engineering and Science Proceedings, 5 (1984) pages 513-529. However, component fabrication by this method is difficult, and the properties of the resulting composites are highly directional.
It had previously been proposed to add relatively large quantities of SiC whiskers to ceramics such as alumina, zirconia, magnesia, ferrites and the like to lower the specific resistance thereof for electric discharge machining. Thus U.S. Pat. No. 4,507,224 describes ZrO.sub.2 ceramics containing as much as 50 weight percent of high-aspect-ratio SiC whiskers for this use. However, as noted in U.S. Pat. No. 4,543,345 relating to whisker-toughened Al.sub.2 O.sub.3 ceramics, no evidence for any toughening effect through the addition of SiC whiskers to ZrO.sub.2 -based ceramic products had been found.
It is therefore a principal object of the present invention to provide ceramic products made of ZrO.sub.2 which exhibit improved toughness for high temperature structural applications.
Other objects and advantages of the invention will become apparent from the following description thereof.